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Abstract We revisit the information on the two lightest a₀ a0 resonances and S -wave πη scattering that can be extracted from photon–photon scattering experiments. For this purpose we construct a model for the S -wave photon–photon amplitudes which satisfies analyticity properties, two-channel unitarity and obeys the soft photon as well as the soft pion constraints. The underlying I=1 hadronic T -matrix involves six phenomenological parameters and is able to account for two resonances below 1. 5 GeV. We perform a combined fit of the γγ→πη and KSKS γγ→KSKS high statistics experimental data from the Belle collaboration. Minimisation of the ² χ2 is found to have two distinct solutions with approximately equal ² χ2. One of these exhibits a light and narrow excited a₀ a0 resonance analogous to the one found in the Belle analysis. This however requires a peculiar coincidence between the J=0 J=0 and J=2 J=2 resonance effects which is likely to be unphysical. In both solutions the a₀ (980) a0 (980) resonance appears as a pole on the second Riemann sheet. The location of this pole in the physical solution is determined to be m-i /2=1000. 7^+12. 9-₀. ₇ -i\, 36. 6^+12. 7-₂. ₆ m-iΓ/2=1000. 7-0. 7+12. 9-i36. 6-2. 6+12. 7 MeV. The solutions are also compared to experimental data in the kinematical region of the decay ⁰ η→π0γγ. In this region an isospin violating contribution associated with ^+ ^- π+π- rescattering must be added for which we provide a dispersive evaluation.
Lu et al. (Fri,) studied this question.